1. Field of the Invention
This invention relates to a zoom lens, and particularly to a so-called four-unit zoom lens in which a second unit for focal length change is comprised of two lens units exhibiting a floating operation in which the amounts of movement resulting from a focal length change are made to differ from each other and is utilized to correct the fluctuation of aberrations resulting from the focal length change. The F number at the wide angle end is as great as 1.75, and the variable power ratio is as high as 16. The zoom lens to which the current invention is directed has good optical performance over an entire variable power range and is suitable for a television camera, a photographic camera and a video camera.
2. Related Background Art
Zoom lenses having a great aperture, high variable power and high optical performance have heretofore been required for television cameras, photographic cameras and video cameras. Among these, in color television cameras for broadcasting, importance is attached to operability and mobility, and in response to such requirements, a CCD (solid state image pickup device), having achieved as high as two millions of pixels per 2/3 inch, has recently been announced as an image pickup device. This CCD has substantially uniform resolving power over the entire image pickup range and therefore, it is required of a zoom lens using it that the resolving power be substantially uniform from the center of the image field to the periphery of the image field.
Of zoom lenses, a so-called four-unit zoom lens comprising, in succession from the object side, four lens units, i.e., a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power for focal length change, a third lens unit of positive or negative refractive power for correcting image plane fluctuating with a focal length change, and a fourth lens unit of positive refractive power for imaging, relatively easily achieve higher variable power and a greater aperture ratio and therefore are often used in color television cameras for broadcasting.
Of four-unit zoom lenses, a four-unit zoom lens having an F number of the order of 1.6 to 1.8, and high variable power ratio of the order of 13 is proposed, for example, in Japanese Laid-Open Patent Application No. 54-127322. Also, a four-unit zoom lens in which a second lens unit for focal length change is divided into two lens units of negative refractive power and in which during a focal length change, the spacing between these two lens units is changed to thereby correct any aberration fluctuation resulting from the focal length change is proposed, for example, in Japanese Laid-Open Patent Application No. 7-13075.
To obtain a great aperture ratio (an F number 1.7 or greater) and a high variable power ratio (a variable power ratio 13 or greater) and high optical performance over an entire variable power range in a zoom lens, it is necessary to appropriately set the refractive power and lens construction of each lens unit.
Generally, to obtain small aberration fluctuation and high optical performance over an entire variable power range, it becomes necessary to increase, for example, the number of lenses in each lens unit to thereby increase the degree of freedom in aberration correction. Therefore, an attempt to produce a zoom lens of a great aperture ratio and a high variable power ratio unavoidably gives rise to the problem that the number of lenses is increased and the entire lens system becomes bulky.
Also, regarding imaging performance, the fluctuation of the point at the center of the image field at which the image contrast is best, i.e., the so-called best image plane, resulting from a focal length change poses a problem. This is attributable chiefly to the fluctuation of spherical aberration resulting from a focal length change.
Generally, the fluctuation of spherical aberration resulting from a focal length change, when the zoom ratio is defined as Z and the focal length of the wide angle end is defined as fw, becomes such as shown in FIG. 57 of the accompanying drawings wherein spherical aberration tends to be under (minus) with respect to the Gaussian image plane from the wide angle end to a zoom position fwm=fw.times.Z.sup.1/4 or fm=fw.times.Z.sup.1/2. When the vicinity of the zoom position fwm or fm is passed, the under amount becomes smaller and becomes 0 at a certain zoom position, and now tends to become over (plus). In the vicinity of a zoom position fd at which F drop in which F number becomes greater (the lens system becomes darker) begins, the under amount becomes most over (plus), and when this zoom position is passed, the over amount becomes smaller toward the telephoto end, and becomes substantially 0 at the telephoto end.
FIGS. 58 to 62 are illustrations showing the states when a light beam passes through a lens system from a first lens unit (F) to a third lens unit (C) at the wide angle end fw, the intermediate fwm (=fw.times.Z.sup.1/4 or so), the intermediate fm (=fw.times.Z.sup.1/2), the zoom position fD of F drop and the telephote end ft of a four-unit zoom lens.
As shown in these figures, the incidence height of an on-axis ray onto the compensator C suddenly becomes lower from the wide angle end fw to the focal length fwm=fw.times.Z.sup.1/4 or the zoom position fm=fw.times.Z.sup.1/2. The incidence height becomes higher to the focal length fd near F drop, and becomes lower again at the telephoto end ft due to F drop. On the other hand, the height of the on-axis ray in the variator V gradually becomes higher from the wide angle end fw toward the telephoto side, and becomes highest at the focal length fd near F drop, and becomes lower at the telephoto end ft due to F drop. This can be summed up as shown in Table 1 below.
TABLE 1 ______________________________________ Zoom Position wide angle telephoto end fw fwm:fm fd end ft ______________________________________ variator V ##STR1## ##STR2## ##STR3## compensator C ##STR4## ##STR5## ##STR6## spherical 0 under over 0 aberration ______________________________________ ##STR7## - ##STR8##
In the compensator C, basically spherical aberration is undercorrected. When in the relay lens unit, spherical aberration is corrected so as to become substantially 0 at the wide angle end, the influence of the change in the onaxis ray in the compensator C is great at the zoom positions fwm and fm on the wide angle side and the height of the onaxis ray in the compensator C becomes lower relative to the wide angle end and therefore, spherical aberration fluctuates to under. When in the fore lens unit, spherical aberration is corrected so as to become substantially 0 at the telephoto end, the height of the onaxis ray in the variator V and the compensator C becomes higher at the zoom position fd of F drop relative t the telephoto end and therefore, the highorder component of spherical aberration fluctuates to over.
On the other hand, even when the third lens unit (compensator) has a positive refractive power, the fluctuation of spherical aberration resulting from a focal length change is similar to that when the third lens unit (compensator) has a negative refractive power, and is summed up in Table 2 below.
TABLE 2 ______________________________________ Zoom Position wide angle telephoto end fw fwm:fm fd end ft ______________________________________ variator V ##STR9## ##STR10## ##STR11## compensator C ##STR12## ##STR13## ##STR14## spherical 0 under over 0 aberration ______________________________________ ##STR15## - ##STR16##
It differs from the case where the compensator is of negative refractive power (not shown) in that the light beam becomes convergent from the compensator to the relay lens unit, but the aberration corrected situations of the variator and compensator conforming to the focal length change are similar.
When in the relay lens unit, spherical aberration is corrected so as to become substantially 0 at the wide angle end, basically spherical aberration is undercorrected in the compensator and therefore, the influence of the change in the onaxis ray in the compensator is great at the zoom positions fwm and fm on the wide angle side, and the height of the onaxis ray in the compensator becomes high relative to the wide angle end fw and therefore, spherical aberration fluctuates to under. When in the fore lens, spherical aberration is corrected so as to become substantially 0 at the telephoto end, the height of the onaxis ray in the variator of particularly strong negative refractive power relative to the telephoto end becomes high at the zoom position fd of F drop and at this time, in the compensator as well, the height of the onaxis ray becomes greatest and due to the influence of the cemented surface of a concave lens used for the aberration correction solely by the compensator, the highorder component of spherical aberration fluctuates to over.
These fluctuations of spherical aberration become more remarkable as the refractive powers of the variator V and the compensator C become greater when the refractive power of the compensator is both positive and negative.
Particularly recently, it has been attempted to strengthen the refractive power of each lens unit and achieve a zoom lens from the desire for compactness and lighter weight and the wider angle of view or higher variable power of the zoom lens. Above all, in the fourunit zoom lens, th refractive powers of the variator, which is a second lens unit, and the compensator, which is a third lens unit, are strengthened and their amounts of movement are decreased to thereby achieve downsizing of the entire zoom lens system and therefore, there has been the tendency that the burden of those movable lens units for aberration correction increases.
Particularly in the case of a zoom lens like a zoom lens for broadcasting of which a high specification and high performance are required, a variator V and a compensator C each are comprised of a combination of at least one negative lens and a positive lens. In addition, a divergence surface for the correction of spherical aberration by a cemented lens is provided or a difference in the refractive index of a medium is provided to correct the aberrations in the interior of each lens unit.
However, the correction of the fluctuation of spherical aberration resulting from a focal length change and highorder chromatic aberration o the like is insufficient and therefore, the number of lenses has been increased or the refractive power of each lens unit has been weakened. Therefore, it has been very difficult to achieve compactness and higher performance of the zoom lens.
In contrast, in the proposition by the aforementioned Japanese LaidOpen Patent Application No. 7-13075, the diameter of the fore lens is made paraxially small and therefore, a second lens unit for focal length chang is divided into two lens units of negative refractive power and the spacing therebetween is only changed in conformity with a focal length change and no mention is made about the effect in aberration correction.